1. Field of the Invention
The present invention relates to ceramic heat exchangers having excellent heat exchange efficiency, small pressure loss and excellent thermal shock resistance, and the invention also relates to a production process thereof.
2. Related Art Statement
As rotary regenerative ceramic heat exchangers having excellent thermal shock resistance, NGK Insulators, Ltd. formerly disclosed a process capable of producing heat exchangers in Japanese patent publication No. 61-51,240 (U.S. Pat. No. 4,304,585 and U.S. Pat. No. 4,357,987), wherein matrix segments are bonded together with a ceramic-bonding material having almost the same composition as that of the matrix segments, and a difference in coefficient of thermal expansion between the bonding material and the segments is not more than 0.1% at 800.degree. C.
Further, Japanese patent publication No. 47-14,838 (U.S. Pat. No. 3,634,111) discloses a foamable cement consisting mainly of Li.sub.2 O, Al.sub.2 O.sub.3, SiO.sub.2, and TiO.sub.2 as an example of the ceramic-bonding material.
In addition, U.S. Pat. No. 4,335,783 discloses a process for producing heat exchangers in which bonded portions are discontinuous.
On the other hand, in the case of Japanese patent publication No. 61-51,240 (U.S. Pat. No. 4,304,585 and U.S. Pat. No. 4,357,987), heat exchanger having considerably improved thermal shock resistance can be obtained. However, since the rotary regenerative heat exchanger requires high thermal shock resistance, the matrix segments themselves must exhibit low thermal expansion, and it is also necessary to employ the bonding material exhibiting low thermal expansion. Consequently such heat exchangers possess problems in that the bonding material exhibiting low thermal expansion is greatly shrunk during firing, that variations in bonding strength are great, and reliability of the bonded portion is deteriorated.
In the case of the foamable cement disclosed in Japanese Patent publication No. 47-14,838 (U.S. Pat. No. 3,634,111), bonding materials exhibiting low heat expansion and small shrinkage can be obtained However, when cordierite matrixes are bonded by using this foamable cement, the foamable cement is insufficiently crystallized during firing for the bonding. That is, the cement reacts with cordierite, so that the coefficient of thermal expansion of the bonded portions increases, and a dimensional change and a change in heat expansion during isothermic aging at high temperatures are great. Consequently, the ceramic matrix product will unfavorably be broken at the joined portions during practical use.
In the case of the discontinuous bonding process described in U.S. Pat. No. 4,335,783, the entire heat exchanger is designed to have a flexible structure and to lessen a difference in temperature between the bending material and the segments by discontinuous bonding so that propagation of cracks is prevented to improve thermal shock resistance. However, strength of the partially bonded portions is unfavorably lower than that of the entire bonded product in the case of large size bonded product.